1. Field of the Invention
This invention relates to a circuit for cancellation of reset noise and, more specifically, to such a circuit, generally in conjunction with a focal plane array (FPA).
2. Background and Brief Description of the Prior Art
Staring focal plane arrays are composed of many detectors which detect a particular portion of a scene and which must be read out in some ordered manner. This read out generally is accomplished by multiplexing the outputs of several detectors into a single preamplifier. In order to obtain accurate reading of the detector outputs at the preamplifier input, it is necessary that, between the serial readings of the detector outputs, the preamplifier input be cleansed of the total signal from the prior sample video, this generally being accomplished by resetting the preamplifier input to a predetermined DC voltage. The reset operation gives rise to a noise term at the preamplifier input generally known as KTC noise which is V.sub.n =(KT/C.sub.p).sup.1/2, where V.sub.n is the noise voltage, K is Boltzmann's constant, T is the temperature in degrees Kelvin and C.sub.p is the input capacitance of the preamplifier in farads and is the sum of all capacitances on the input node, preamplifier capacitance, read line capacitance and stray capacitance. This noise term can be substantial compared to the signals produced by the detectors of the array and thereby introduce significant error in the output. Accordingly, this noise must be cancelled out to fully realize the performance potential of the focal plane array.
The above described reset noise remains correlated from the reset time to the read time for the next detector and can be removed by a correlated double sampling. However, the magnitude of the noise term changes when the detector is addressed. This is better understood with reference to FIGS. 1 and 2. When the preamplifier input is reset as shown in FIG. 2 for timing signal .phi..sub.R, switch S.sub.R closes, switches S.sub.1 through S.sub.N, which represent the detector addresses, being open at this time. This closing and opening of switch S.sub.R provides the undesirable KTC noise. The detectors are schematically represented in all figures herein as capacitors C.sub.D1 to C.sub.DN charged to voltages representing the video information detected by the associated detector. The noise created by the reset is stored on the clamp capacitor C.sub.c before the detectors are addressed. The switch S.sub.c is then closed as shown in FIG. 2 for timing signal .phi..sub.c to discharge capacitor C.sub.c and store the KTC noise value thereon. The switch S.sub.N is then closed as shown in FIG. 2 for timing signal .phi..sub.DN to transfer the charge on capacitor C.sub.DN to capacitor C.sub.p. The signal voltage is amplified by the preamplifier and is coupled through the capacitor C.sub.c. The signal is transferred to capacitor C.sub.s at time .phi..sub.s. (The gain of the preamplifier is assumed to be unity for ease of explanation. Also DC terms are not carried through for the same reason.)
When switch S.sub.N closed, a charge representing the video from the detector and stored on the capacitor C.sub.DN is stored onto capacitor C.sub.p along with the noise term thereat. The result with regard to the noise is that the noise on the capacitor at the preamplifier input drops to: EQU V.sub.n '=[C.sub.p /(C.sub.p +C.sub.DN)](KT/C.sub.p).sup.1/2
The noise that appears on the sample (i.e., the capacitor C.sub.s when O.sub.s is closed) is: EQU V.sub.n.sup.o =V.sub.n '-V.sub.n =(KT/C.sub.p).sup.1/2 [C.sub.p /(C.sub.p +C.sub.DN)-1]
It is readily apparent that except where C.sub.p is much greater than C.sub.D, the cancellation scheme is not effective.
The same problem has been solved in barium strontium titanate (BST) focal plane arrays by using paralleled amplifiers to drive the correlated double sampler (CDS) clamp capacitor C.sub.c. The gains were scaled such that the difference in gains matched the capacitive divider and rejection was achieved. Another prior art scheme changes the gain of the preamplifier from clamp to sample time in order to track the capacitive divider at the input.
Some problems with these solutions are:
1. For parallel amplifiers, additional power dissipation is required whereas space for the additional circuitry is at a premium for IR focal plane arrays (FPAs). Also, 1/F noise from the parallel amplifiers does not tend to cancel and, in fact, may be additive.
2. Sequential gain changing is time consuming and provides a technical challenge to maintain tracking from channel to channel. The more complex circuits consume additional power. DC shifts in the preamplifier output also corrupt the signal from clamp to sample time.